Field of the Invention
The present invention relates to a communication apparatus having an IPFAX function, a control method therefore, and a computer-readable storage medium storing a control program for implementing the method.
Description of the Related Art
In recent years, IPFAX capable of sending and receiving faxes using an IP network has been receiving attention. Contents for an NGN (Next Generation Network) network by NTT (Nippon Telegraph and Telephone Corporation) also include IPFAX. For call connection to IPFAX, the SIP (Session Initiation Protocol) that is a session control protocol is mainly used.
On the other hand, in the present PSTN (Public Switched Telephone Network) network, analog data is communicated using FAX modems. A typical example of FAX modems is an AT modem. The AT modem is of a type that provides modem control using the AT command set. The AT command set is a command set for modems, which was proposed by Hayes Communications Inc. in the U.S. and is the most popular at present. Commands beginning with “AT” enable overall sending and receiving functions of a modem to be easily operated from a controller.
The FAX protocol T.38 used for IPFAX is required to comply with the FAX protocol T.30 for PSTN. For this reason, FAX application resources for PSTN so far can be used substantially as they are. However, regarding call connection to IPFAX, many modifications are made because of a change to SIP. Although there is the technique to convert the AT command set into a command set complying with the T.38 protocol, no mention is made of call connection SIP (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2001-197280).
For example, assume that an AT command issued from a FAX application is converted into a SIP command by an AT-IPFAX command conversion unit, and call connection is attempted by an IPFAX module. In an AT modem, an AT command for dialing is “ATD”. For tone dial, “T” is appended to it, and for pulse dial, “P” is appended to it. At the end, a telephone number is added like “ATDT04412345678”, and this command is issued to the IPFAX module. The AT-IPFAX command conversion unit in the IPFAX module converts the ATDT command into an INVITE command for requesting call connection (session establishment) using SIP. After that, a session control unit in the IPFAX module attempts INVITE connection using SIP. After receiving the ATDT command, the AT-IPFAX command conversion unit has to return a result code to the FAX application. With this result code, the FAX application determines that dialing has been completed and starts a T0 timer. When the timing with which this result code is returned to the FAX application cannot be accurately controlled, the FAX application cannot exactly know the timing with which dialing is completed, and hence measurement by the T0 timer cannot be accurate.
The T0 timer is supposed to be started after dialing is completed. Both an AT command and a SIP command may include the concept that a command ends, but they do not have the concept that dialing ends. For this reason, if the T0 timer is started with end of a command, the problem that the measurement by the T0 timer cannot be accurate will arise.
When sending of a calling tone (CNG) is requested from the FAX application after dialing is completed, the AT-IPFAX command conversion unit converts this request into a T.38 CNG command which complies with the T.38 protocol. Thereafter, a T.38 control unit in the IPFAX module sends out the T.38 CNG command to a network server or directly to an opposing machine. It is, however, useless to send out a T.38 CNG command before a SIP session is established, and hence it is necessary to control the timing of the sending as well.